A metal catalyst may be supported in small cavities of a carrier base material. For example, in a fuel cell, an electrode may be made of an electrically conductive material having small cavities serving as a diffusion path of a reactive gas, and platinum as a metal catalyst for accelerating an electrochemical reaction may be supported in the cavities of this electrically conductive material. Techniques described in Patent Literatures 1 and 2 given below have been proposed to support the metal catalyst in the small cavities of the carrier base material. The technique disclosed in Patent Literature 1 or Patent Literature 2 disperses a platinum complex solution in supercritical carbon dioxide and supports platinum on a carbon nanotube substrate or a porous substrate placed in the supercritical carbon dioxide.